PROJECT SUMMARY The role of the Follicle Stimulating Hormone Receptor (FSHR) in female fertility is thought to be restricted to the known actions of ovarian FSHR. Recently published studies from my laboratory challenge this dogma by demonstrating extra-ovarian FSHR expression in the female reproductive tract of cycling and pregnant women and the placenta and decidua of pregnant women, and unequivocally demonstrating a critical role for placental FSHR in pregnancy. The focus of the present studies is the physiological role of myometrial FSHR in female fertility. We show that FSHR is expressed in myometrium and that during pregnancy it is significantly up-regulated at term. Notably, we demonstrate that FSH promotes myometrial quiescence in the non-pregnant state and in early pregnancy when FSHR levels are low, but it stimulates contractile activity at term pregnancy when FSHR levels are high. Although maternal pituitary FSH is suppressed during pregnancy, our published studies suggest that FSH is synthesized in the placenta, decidua, and myometrium, where it can act as a paracrine and/or autocrine signal to engage the myometrial FSHR. We hypothesize and will test herein that myometrial FSHR contributes in a significant manner to both the establishment and the maintenance of pregnancy as well as the timing of parturition. Specifically, by promoting uterine quiescence early in pregnancy, we postulate that myometrial FSH/FSHR signaling is required to silence uterine contractions and thereby promote embryo implantation and the subsequent maintenance of pregnancy early in gestation. At the end of gestation, when myometrial FSHR expression is up-regulated, we postulate that FSH/FSHR signaling is required for increased contractile activity and thereby contributes to the timing of parturition. As an exploratory R21 designed to provide proof-of-principle that myometrial FSHR contributes to fertility and the successful completion of pregnancy, we will test our hypothesis utilizing a novel and powerful mouse model in which CRISPR/Cas9 was used to insert loxP sites flanking exon 10 of the Fshr to enable the conditional deletion of functional FSHR from myometrium. If the findings support our hypothesis, the outcomes would be paradigm shifting with respect to our understanding of FSH/FSHR signaling in female reproduction. It is particularly relevant to consider that several studies suggest that the risks of failed implantation, spontaneous miscarriage, and adverse perinatal outcomes in infertile women undergoing Assisted Reproductive Technologies (ART) are not necessarily due to ART, but may be due to maternal factors underlying infertility. If one such factor is a suboptimal response of ovarian FSHR to FSH, it follows that a similarly suboptimal response of myometrial FSHR to FSH may contribute to an inability to achieve or properly sustain pregnancy. The results of the proposed studies may therefore spur the development of novel therapeutic approaches based on myometrial FSHR to address infertility and poor pregnancy outcomes.